This invention pertains to seal housings, for use in rotary machines such as pumps or liquid mixers, and the like, and in particular to seal housings for mechanical face seals arranged between the hydraulic components of the machine and the driving motor therefor, as is typical in pumps of the centrifugal type.
A conventional centrifugal pump comprises a driving unit, such as an electric motor, and a pump housing connected via a driving shaft. In order to prevent the pumped medium from entering the electric motor via the shaft, and cause damage, some sort of seal is arranged between the pump housing and the motor. A typical seal is the so-called mechanical face seal which consists of a rotary seal ring and a stationary seal ring which are pressed towards each other by a biasing force.
Seals of the aforesaid type are very effective, providing that the two seal surfaces are even and undamaged. In order to effect a perfect sealing, there is often used an arrangement consisting of two mechanical seals with an intermediate, intervening oil chamber. In this way, one of the seals will always be lubricated and cooled by clean oil, and the risk of damage is greatly reduced. Of the two seals, the one which faces the pump housing, here called the lower seal, will always be exposed to the pumped medium, and damage can result if the pumped medium contains abrasive particulate. The latter is a circumstance which is common in mines and certain other industrial areas.
The lower seal is normally mounted in a cavity which has a cylindrical or conical form. If conical, the portion with the greater diameter confronts the pump housing. The conical form has manufacturing purposes, but the shape is also used on the theory that particles, which enter the cavity, will be thrown out again by centrifugal force. In fact, however, pollutions in the form of particles having higher density than water are concentrated at the inner part of the cavity where they may damage the seal surfaces as well as the cavity wall. This phenomenon also occurs in seal housings where the cavity has a cylindrical-formed shape.
Concentration of pollutants proceeds from the fact that rotation of the pumped medium generates secondary flows of such a nature that rotating surfaces induce a radially outwards-directed flow, while stationary surfaces induce a radially inwards-directed flow. Particles within the seal housing will be thrown outwards to the cylindrical or conical wall surface by centrifugal force, the boundary layer flow along the wall surface then directing the particles within the housing toward the seal.
The particles are not transported back to the pump impeller, at the outlet of the seal housing, as the backwards-directed flow is weaker than the inwards-directed flow, and because of the fact that the centrifugal force again throws the particles towards the inwards-directed boundary layer flow. The result is that particles which once have entered the seal housing will never leave it, but are concentrated in the inner part of the housing where they cause wear of the housing and the seal.
In order to diminish the risk of wear of the seal ring surfaces and the seal housing, there have been prior art attempts to change the flow pattern in the seal area. An example thereof is shown in U.S. Pat. No. 4,872,690, where an additional means is mounted which turns the flow and thus sometimes solves the problem.
Another solution is shown in U.S. Pat. No. 5,513,856 wherein a spiral formed land in the seal cavity makes the particles move in a direction away from the seal. The boundary layer flow still tries to bring the particles inwards and toward the seal, but the land helps the centrifugal force to bring the particles out of the boundary layer flow and keep them within the land.
The instant invention is a further development of the last mentioned solution in which the wall of the seal housing is configured in a more effective arrangement.